greybus: reserve host cport id 0

For ES1 and ES2, we use pad bytes in an operation message header to
encode the CPort ID used for transferring the message.  The pad
bytes should otherwise be zero, and we ensure this as the message
is passed to or from the upper layer.

If host-side CPort ID 0 is used, we have no way of knowing whether
the CPort field has been "packed" into the header.

To allow detection of this, reserve host CPort id 0.  Update
cport_id_valid() to treat 0 as invalid.

(CPort ID 0 is reserved by one of the UniPro standards.  We'll
assume for now that we never use it for Greybus.)
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/core.c

@@ -170,6 +170,7 @@ struct greybus_host_device *greybus_create_hd(struct greybus_host_driver *driver
 					      size_t buffer_size_max)
 {
 	struct greybus_host_device *hd;
+	int ret;
 
 	/*
 	 * Validate that the driver implements all of the callbacks
@@ -200,12 +201,19 @@ struct greybus_host_device *greybus_create_hd(struct greybus_host_driver *driver
 	if (!hd)
 		return ERR_PTR(-ENOMEM);
 
+	ida_init(&hd->cport_id_map);
+	/* Reserve CPort id 0 */
+	ret = ida_simple_get(&hd->cport_id_map, 0, 1, GFP_KERNEL);
+	if (ret < 0) {
+		kfree(hd);
+		return ERR_PTR(ret);
+	}
+
 	kref_init(&hd->kref);
 	hd->parent = parent;
 	hd->driver = driver;
 	INIT_LIST_HEAD(&hd->interfaces);
 	INIT_LIST_HEAD(&hd->connections);
-	ida_init(&hd->cport_id_map);
 	hd->buffer_size_max = buffer_size_max;
 
 	return hd;

drivers/staging/greybus/greybus.h

@@ -198,7 +198,7 @@ static inline int is_gb_connection(const struct device *dev)
 
 static inline bool cport_id_valid(u16 cport_id)
 {
-	return cport_id != CPORT_ID_BAD && cport_id <= CPORT_ID_MAX;
+	return cport_id && cport_id != CPORT_ID_BAD && cport_id <= CPORT_ID_MAX;
 }
 
 #endif /* __KERNEL__ */